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Bernie Sanders vowed to win primaries in California and beyond even as Hillary Clinton secured the commitments of enough delegates to become the presumptive Democratic presidential nominee.

Campaigning in San Francisco, Mr Sanders declined to speculate to reporters about what a poor showing in Tuesday's primaries might mean to his presidential campaign.

"Let me just talk to you after the primary here in California where we hope to win. Let's assess where we are after tomorrow," he said.

Later, at an evening rally in Golden Gate Park, Mr Sanders steered clear of reports from The Associated Press that Mrs Clinton had reached the 2,383 delegates needed to become the presumptive Democratic nominee.

His spokesman Michael Briggs said Mrs Clinton's support was dependent upon superdelegates who could still change their minds between now and the July convention.

He said the campaign's job is to convince the superdelegates that the Vermont senator is "by far the strongest candidate against Donald Trump".

Mr Sanders also faced new questions about the future of his campaign amid reports that President Barack Obama was readying an endorsement of Mrs Clinton.

Mr Obama called Mr Sanders on Sunday as he campaigned in California, a Democrat source told The Associated Press.

Asked if he had talked to Mr Obama, Mr Sanders said. "I have spoken to President Obama many, many times about many issues, and I really think it's not appropriate to talk about my discussions with the president. I try to keep that private."

Hillary Clinton

Democrats are voting in six states on Tuesday, headlined by California, the nation's largest state, offering 475 pledged delegates, while Mrs Clinton, a former New York senator, is heavily favoured in New Jersey primary, which has 142 pledged delegates.

Mr Sanders, a self-described democratic socialist, who has won 20 states, has outlined plans to influence the party platform and try to persuade superdelegates that he would fare better than Mrs Clinton against presumptive Republican nominee Mr Trump.

Mr Sanders has previously said that Mrs Clinton should not be deemed the party's nominee because she would be relying on superdelegates - party officials and elected leaders - who do not actually vote until the Democratic National Convention.

While Mrs Clinton has been in the driver's seat for the nomination for weeks, a victory by the frontrunner in California would give Mr Sanders much less leverage as he seeks to sway superdelegates.